Fix PEP 249 compliance

[Vnajeb]

Fixes #91

Changes proposed in this pull request:

• Fixed cursor.description to return a sequence of 7-item column tuples (name, type code, display size, precision, scale, nullable) after query execution, mapped from the server's QueryMetaData.columns
• Fixed fetchone(), fetchall(), and fetchmany() to return row tuples instead of raw response dicts
• Applied the same fixes to AsyncCursor
• Fixed rowcount to be consistently set after execute and executemany across both cursors
• Added new unit tests for cursor PEP 249 compliance that run without a live server using mocked WebSocket responses

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